Past exam of the chemistry course of the year 4 of the natural sciences course of the University of Cambridge / 2022 / Paper 1 / 13 / a

One standard description of sodium-rich NaCl is removal of lattice chloride into a NaCl reservoir, leaving a positively charged chloride vacancy and an electron:This Kröger–Vink notation equation conserves atoms, chloride sites and effective charge. The electron can be trapped in the vacancy, forming an electrically neutral F center:Thus sodium excess introduces electronic defects and can give coloration. It does not imply that every added electron is a mobile conduction electron: electronic transport depends on trapping and thermal release. Chloride vacancies can also assist anion transport. An alternative microscopic incorporation route is a sodium interstitial plus an electron, but the vacancy–electron model describes the usual alkali-metal-colored halide.

The trivalent dopant is comparable in radius to Na${}^{+}$ and is naturally accommodated on a sodium site rather than a chloride site. One charge-balanced incorporation reaction isThree cation sites are filled by one Y ion and two vacancies; the three chloride ions occupy the corresponding anion sites. The effective $+2$ substitutional charge is balanced by two singly negative sodium vacancies.

YCl${}_3$ doping primarily increases sodium-vacancy-mediated ionic conductivity, provided the vacancies remain mobile rather than strongly bound to the dopant. In contrast, sodium-rich doping introduces electrons and anion vacancies or interstitial-related defects. Neither mechanism requires holes as its compensating carrier, and an increase in vacancy number need not translate proportionally into conductivity if defect association reduces mobility.